PROCEDURE 18 • Prone positioning is used as an adjunct short-term supportive therapy in an attempt to recruit alveoli to improve gas exchange in a critically ill patient with severely compromised lungs. • The last major outcomes to be examined in the meta-analyses were the presence of significant complications when the prone position was compared with the supine position. Two of the three analyses reported on complications. One analysis showed a statistically significant higher risk for the development of pressure ulcers in the prone position,28 and Abroung and group1 showed no significant difference in major airway complications in the prone position. All three meta-analyses concluded that an adequately sized study optimizing the duration of proning and ventilation strategy is warranted to be able to draw definitive conclusions. A phase III trial looking at these issues has been completed but the results have not been published to date. To enhance an understanding of how prone positioning may affect gas exchange, understanding the factors that influence the distribution of ventilation and perfusion within the lung is important. • Distribution of ventilation: Regional pleural pressures and local lung compliance jointly determine the volume of air distributed regionally throughout the lungs. Three major factors—gravity and weight of the lung, compliance, and heterogeneously diseased lungs—influence regional distribution. In an upright individual, the pleural pressure next to the diaphragm is less negative than at the pleural apices. The weight of the lung and the effect of gravity on the lung and its supporting structures in the upright position create this difference in regional pleural pressures. This relationship results in a higher functional residual capacity (FRC) in the nondependent zone or the apices, redirecting ventilation to the dependent zone.8,15,35 When body position changes, changes occur in regional pleural pressures, compliance, and volume distribution. In the supine position, distribution becomes more uniform from apex to base. The ventilation of dependent lung units exceeds that of nondependent lung units, however, and a reduction in FRC is seen.8,35 The two factors that contribute to the reduction in FRC seen in moving from the upright to the supine position include: 1, the pressure of the abdominal contents on the diaphragm8; and 2, the position of the heart and the relationship of the supporting structures to the lung and its influence on pleural pressure gradients.17,20 • Distribution of perfusion: Similar to ventilation, regional distribution of perfusion is influenced by three factors: cardiac output, pulmonary vascular resistance, and gravity or body position. • In an upright individual, blood flow decreases as it moves from base to apex with virtually no flow at the apex. This decrease is caused by the influence of gravity on pulmonary vascular pressures within the lung (Fig. 18-1). Figure 18-1 Zone model of the lung. Three-zone model of the lung is used to explain distribution of blood flow based on pressure variations. (From West JB, Dollery CT, Naimark A: Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures, J Appl Physiol, 19:713-724, 1964.) • In supine and lateral positions, apical region blood flow changes. No real change is seen in basilar units, but a greater dependent versus nondependent blood flow occurs. In the prone position, a marked reduction occurs, however, in the gravitational perfusion gradient, which suggests no gravity-dependent benefit to flow in the prone position.21 • On the basis of the current available data as outlined here, changes in oxygenation seem to be related to differences in the regional inflation/ventilation of the lung while prone and are not related to a redistribution of blood flow.6,17,23 • Suggested criteria for use of the prone position include: • Contraindications and precautions to manual pronation therapy include the following3,7,23,26,31,32:
Pronation Therapy
PREREQUISITE NURSING KNOWLEDGE
On the basis of numerous studies and three recent meta-analyses, patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) placed in the prone position significantly increase partial pressure of arterial oxygen (PaO2) to fraction of inspired oxygen (FiO2) when compared with the supine position. The greatest effect was seen within the first day, with continuing benefit with subsequent prone position placements up to 4 days.1–5,7,9,10,13,18,22,24,28,30,32
Two of the three meta-analyses showed no improvement in mortality with the use of the prone position.1,28 One showed significant improvement in mortality in patients with severe ARDS and a higher severity of illness.2
No significant difference was seen in number of days on mechanical ventilation with the prone position.1,2,28 One meta-analysis showed significant reduction in the incidence of ventilator-associated pneumonia (VAP) in the prone position28; another showed a trend toward significance in VAP reduction of 23% (P = 0.09)1; and the third showed no difference in VAP rates between the two positions.2
Part of the variability between the analyses has to do with the inclusion criteria used to choose the studies incorporated in the meta-analysis. The analysis by Alsaghir and Martian2 resulted in five studies that met inclusion criteria out of 63 with a total of 1316 patients. The meta-analysis preformed by Abroung and group1 included 5 trials out of 72 with a total of 1372 patients, and the analysis by Sud and colleagues28 included 13 trials out of 1676 studies with analysis being performed on 1559 patients.
The first factor to influence pleural pressure, regional volumes, and FRC is the impact of the abdominal contents on the function of the diaphragm. In spontaneously breathing individuals in the supine position, the diaphragm acts as a shield against the pressure exerted by the abdominal contents, preventing the contents from interfering with dependent lung volume distribution. When patients are mechanically ventilated with positive-pressure breaths, sedated, or paralyzed, the active muscle tension in the diaphragm is lost, which results in a cephalad displacement of the diaphragm and allows abdominal pressures to decrease dependent lung volume inflation and FRC.8,15 The only way to modify this influence is to change the posture to a prone position with the abdomen unsupported.8,24
The second factor to influence pleural pressure, regional volumes, FRC, and compliance is the position of the heart and supporting structures. The heart and the diaphragm extend farther dorsally and rest against a rigid spine in the supine position, squeezing the lungs beneath them. This pressure on the lungs generates more positive pleural pressures, which results in a greater propensity to collapse of the alveoli at end expiration. In the prone position, the heart and upper abdomen rest against the sternum, exerting less weight on the lung tissue. Less effect on pleural pressure occurs, which leaves the pleural pressures more negative, maintaining open alveoli.17,21,24
A third factor that contributes to the distribution of volume is heterogeneously or unevenly distributed diseased lung. The acute respiratory distress syndrome lung weight is increased twofold to threefold from normal. The increased weight is from edema and the resulting hydrostatic forces. A progressive squeezing of gas along a vertical-dorsal axis results. This decrease of regional inflation along the vertical axis results in dependent or dorsal lung collapse. In the prone position, these densities shift. The pattern almost completely reverts toward normal. The inflation gradient is less steep, and the difference results in a more homogeneous regional inflation. This inflation may be related to a redistribution of gas because of the change in hydrostatic forces caused by differences in pleural pressure, as described previously.9,11,24
In zone 1, near the apex, alveolar pressure exceeds arterial pressure, creating little or no flow.
In zone 3, the arterial pressure is greater than the venous pressure, which is greater than the alveolar pressure. In this zone, the influence of the alveolar pressure on blood flow is reduced, resulting in freedom of flow in this region.35,36
Consider use of the prone position for patients with ARDS who need potentially injurious levels of FiO2 or plateau pressure, provided they are not put at risk from positional changes6
Patient unable to tolerate a head-down position
Increased intracranial pressure
Unstable spine (unless Stryker Frame [Stryker Medical], Kalamazoo, MI used)
With use of a support frame, patient weight greater than 135 kg
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18: Pronation Therapy
